A mass spectrometer typically comprises an ion source, a mass analyzer, an ion detector and a data system. The ion source contains an ion generator which generates ions from a sample, the mass analyzer analyzes the mass/charge properties of the ions, the ion detector measures the abundances of the ions, and the data system processes and presents the data. In certain ion sources, an ion sampling apparatus is included as an interface to collect and transport ions from the ion generator to the mass analyzer. If both positively and negatively charged ions are produced in the ion source, the ion sampling apparatus needs to transport both kinds of ions. Since positive and negative ions may collide and lose their charges, the transport is usually achieved by switching the polarity of the ions that travel into the sampling apparatus so that only positive ions or negative ions are transported at the same time.
The new developments in liquid chromatography (LC) and ion generation sources have resulted in narrower chromatographic peaks (peak width often less than 2 seconds) and the possibility of identifying a vast variety of chemical compounds from an original sample in a single LC run. In order to identify all or nearly all components, it is important to generate and detect both positive and negative ions during the single LC run. Therefore, in more recent applications, the ion sampling apparatus not just maintains the ion transmission, but also needs to be able to switch the polarity of sampled ions quickly, preferably in less than 100 millisecond time intervals. At present, metal capillaries are used as ion transfer tubes (for example, see U.S. Pat. No. 4,977,320); however in this case the ion generator needs to be operated at high voltages, typically more than 1 kV. Even though these metal capillaries provide fast ion polarity-switching capabilities and are thus compatible with high resolution LC, the user safety issues involved in handling ion sources operated under high voltages make this configuration undesirable. Thus, an improved sampling apparatus is needed for fast polarity-switching ion sources.
Glass capillary sampling devices have been described (see, e.g., U.S. Pat. No. 4,542,293), utilizing non-conductive glass or fused silica material for ion transfer. However, it was discovered later that fused silica capillaries led to unstable ion transmission and dramatic sensitivity drops during continuing operation. Furthermore, these capillaries have only been operated with slow polarity-switching power supplies.